Symetrix 628 - Page 18

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quickly tiring anyone listening for an extended period. The compressor would purge all dynamic
range; only the frequency would vary. Heavy compression with short attack and release timing
made AM radio loud in the 60s, but it drove listeners to FM radio and cassette tape. Listener
fatigue on the AM band killed their audience by robbing their program material of all dynamic
range. Compression becomes an asset when we moderate the rate of correction.
The attack time control tells the compressor how quickly to apply compression after signals rise
above the threshold. You may compare this to how hard you step on the gas pedal in your car
when you want to accelerate. You can take nine seconds to get to 55 mph, or you can step on the
accelerator gently and take several minutes to reach 55 mph.
We state attack time as a function of decibels (of compression) per millisecond; it is a rate of
attack. This describes how much compression is applied per millisecond once the signal crosses
the compressor threshold. Using an attack time of 1.0 dB/mSec results in the application of
compression at a rate of 1 dB every millisecond until the compressor satisfies its ratio and thresh-
old requirements.
Audio containing a wide dynamic range and irregular wave forms (speech and hard rock music)
require short attack timing for tight control. Music sounds more pleasant as longer attack times
allow occasional short duration peaks to pass through without compression. Broadcast final
limiters function with very short attack timing to eliminate modulation overshoots. Audio clipper
circuits produce absolute overshoot protection (a brick wall) with instantaneous attack times.
Release timing tells the compressor how slowly to release compression after responding to a falling
signal peak. It is a rate of release. You may compare this to how firmly you step on your cars
brake pedal when you want to slow down. How quickly do you want to stop?
Release timing is best expressed as decibels (of compression) per second. A release time of 10 dB/
Sec allows the compressor to release compression at a rate of 10 decibels every second. The faster
the release time, the more aggressive the compressor operation.
Speech, with its irregular wave forms, can tolerate faster release than music which contains
smoothly shaped sine waves. Use short release timing for aggressive processing, but remember
what happened to AM radio. Slower (longer) release timing preserves more dynamic range of the
source audio.
Linear vs. Downward Expanders
Expander operation brings confusion unless its remembered that the expander modifies the
dynamics, the changes, of signals passing through the circuit. An expander converts a slowly
rising audio voltage into a rapidly rising audio signal; it amplifies the change. Expanders come in
two types: linear and downward.
Linear expanders increase the dynamic range of all signals, no matter what their actual level. The
linear expander simply makes all changes greater by some ratio, sometimes user adjustable. In the
real world, linear expanders arent too practical because clipping occurs when signals just below
maximum output level expand upward.
For instance, an unprocessed signal 3 dB below clipping that increases 2 dB wont distort, because
its still 1 dB below maximum. But if that same signal passes through an expander operating at a 1:2
ratio, the same 2 dB change at the expanders input becomes a 4 dB change at its output. That
signal would expand to 1 dB over maximum, causing distortion. Use linear expanders with care,
because very few systems have enough headroom to handle the upward dynamic range which
linear expanders can produce.
The term expander generally describes a downward expander, because the device only expands
signals which drop below a threshold. The downward expander gives the operator the ability to expand
the usable dynamic range of audio passing through the system without running out of headroom.
Note: In the interest of clarity, the term expander will be defined as a downward
expander from this point forward.